Electrokinetic leaching

ABSTRACT

A method of treating a solid material containing a substance of economic importance so as to recover or extract the substance from the material involving the following: forming a conducting mixture of the solid with a liquid so that at least some of the substance of economic importance is dissolved in the liquid, applying an electrical potential difference between at least two electrodes in contact with the mixture at spaced apart locations so as to impart opposite charges to the respective electrodes thereby attracting the liquid containing the dissolved substance to one of the electrodes, removing the liquid from an area surrounding or from a vicinity of the one electrode, and treating the liquid containing the substance of economic importance so as to recover or extract the substance. The liquid used is at least partially conducting. The present method is particularly useful for extracting or recovering gold from gold-containing or gold bearing ores.

The present invention relates generally to the treatment of naturallyoccurring materials having some economic importance, such as for examplemined materials or ores, and particularly to the treatment of orematerials containing a valuable component, such as for example aprecious metal. More particularly, the present invention relates to theextraction of one or more components of a solid material or a mixture orcombination of solid materials by the dissolution of the valuablecomponent or components in a liquid and the subsequent or simultaneousextraction of the dissolved component(s) from the liquid. In particularthe present invention relates to the extraction of precious metals, suchas for example gold, silver and the like from ores containing thesemetals. The present invention finds particular application as animprovement in the extraction of gold from gold containing materialsusing a leaching solution to dissolve the gold from these materials.

Although the present invention will be described with particularreference to an improvement in the leaching step of the overall processof extracting gold from gold-containing materials, such as for exampleores, it is to be noted that the scope of the present invention is notso limited to being restricted to the described embodiment, but ratherthe scope of the present invention is more extensive so as to includeother methods and processes of obtaining materials of economic value andto other applications than those specifically described.

It is further to be noted that although the present invention will bedescribed with particular reference to recovering or extracting gold,the use of this term is merely for the sake of clarity and brevity onlyand is not meant to be limiting to the scope of the invention in anyway. Also, it is to be note that the process methods of the presentinvention are equally applicable to other materials of high economicvalue, such as for example silver, copper and other valuable or preciousmetals.

BACKGROUND OF THE INVENTION

Existing methods for recovering or extracting gold from gold-containingor gold bearing ores consist of three separately identifiable stages.The first stage is the dissolution of the valuable component i.e. thegold, by forming a solid/liquid mixture i.e. a solution of dissolvedgold. The second stage is the separation of the solids and liquids fromthe liquid/solid mixture to produce a "clean" solution of the liquidcontaining the dissolved gold or valuable component. The third stage isthe extraction of the valuable component from the clean solution. Thepresent invention is an improvement in and/or an alternative for thefirst two of these three stages. Thus, the present invention relates toan improvement in the steps of dissolving the gold and separating thedissolved gold solution from undissolved raw material.

Existing methods of recovering or extracting gold from solid materialsusing the above three stages can be generally classified in accordancewith the particle size of the solid material bearing the gold that is tobe treated by the method.

Firstly, for materials having a largest size of about 1 mm and having arelatively low proportion of fines (which is the term used to describeparticles having a size of less than 20 micrometers), one existingmethod consists of forming a slurry of the solid material to be treatedwith a suitable leach solution and agitating the slurry so formed for apredetermined time to allow the gold to more or less fully dissolve. Theundissolved solid material is then separated from the liquid leachsolution by washing, thickening and/or filtering the slurry in order toprovide a "clean" solution containing the gold. The separatedundissolved solid material is then discarded to waste or may beretreated.

Secondly, for materials which have a largest size of about 500micrometers and a large proportion of fines, merely dissolving the solidmaterial in a leach solution is not feasible due to the difficultiesassociated with separating the undissolved solid material from the leachsolution to obtain a clean solution of the gold. In the treatment ofmaterials having this particle size range it is additionally necessaryto add to the slurry relatively large sized particles of an absorbentsubstance or substances which absorb the gold from the liquid and whichcan then be more readily separated from the remaining undissolvednon-gold bearing material and from the leach solution. These relativelylarge sized particles of absorbent materials are subsequently removedfrom the slurry, often by screening, and then subjected to furtherprocessing to extract or otherwise recover the gold.

Thirdly, for material which has a largest size of greater than 1 mm thematerial to be treated may be placed in a vat and then flooded withleach solution. After a predetermined time the gold-containing solutionis drained from the base of the vat providing a clean solution forfurther processing to recover the valuable component.

Fourthly, for material with a largest size of up to two meters thematerial may be placed in a heap and the heap irrigated with leachsolution. The solution permeates the heap and exits from the base of theheap carrying dissolved gold. The gold can then be recovered from theleach solution.

In any of the above described processes the particle size of thematerial being treated may be modified. For example, gold bearingmaterial such as ore, is seldom found in a form where the gold isavailable for direct contact by the leach solution. The more usualsituation is where the gold is encased either partially or wholly in thehost rock which is usually highly impermeable to liquid and thusinitially the leach solution does not have direct access to the gold. Asa consequence, it is common practice to comminute the material to a sizewhere the gold is more accessible and can be contacted by the leachsolution. This process is known as liberation of the gold.

In cases where the solids/liquid separation stage is achieved by meansof drainage, such as in the third and fourth methods described above, itis sometimes necessary to agglomerate the dissolved material prior torecovering the gold which has the effect that any fine particles whichare present are agglomerated along with the large particles of materialbeing treated and accordingly are held stationary within the body of thematerial being leached, thus preventing further recovery.

All of the above processes have one or more limitations., problems orshortcomings and further, they do not always result in the gold beingextracted or recovered efficiently or economically.

The first method as described above is limited to the treatment ofmaterial which can be readily separated to form a clean solution, suchas for example by being readily dewatered by gravitational techniques.Consequently, this process is limited to only those materials of arelatively coarse size only. Additionally this method is also relativelycostly in terms of both the capital expenditure required to initiallyset up the plant and equipment for carrying out this process, and theongoing operating costs.

Whilst the second method overcomes the dewatering problems associatedwith the first method it still remains relatively costly to both set upand operate.

The third and fourth method both rely completely on the natural drainageproperties of the material being treated in order to achievesatisfactory solid/liquid separation. This is not always effective sincethe leach solution containing the gold may accumulate in unwanted or ininaccessible locations. In both the heaps of material being treated andthe vats containing the material being treated, migration of finematerial to either the base of the heap or to a position within the bodyof the material being leached sometimes prevents adequate drainagethrough the heap which creates zones where the leach solution becomesstationary and accordingly the method is inefficient in this respect.

Whilst such problems may be partially overcome by agglomeration of thefines, the formation and strengthening of the agglomerates which issometimes necessary constitutes an additional step in the overallprocess which further adds to the overall cost of the process.

Therefore, there is a need for a material treatment process in which theleach solutions can be made to pass more efficiently through a widevariety of materials having virtually any particle size distribution.Accordingly, it is one aim of the present invention to provide a processcomprising a more efficient leaching step which is applicable to a widerange of materials, irrespective of the properties of the material andwhich is an improvement over existing methods. The improvement isachieved by applying a potential difference across the solid/liquidmixture so that by the use of electro-osmosis or similar processesliquid is caused to pass through the solid/liquid material at a ratesignificantly greater than that which can be achieved undergravitational forces alone.

Electro-osmosis as the term is used in the present specification is aterm used to describe the phenomenon caused when an electrical potentialdifference is applied across a solid/liquid mixture. The solid particlespresent in the slurry or liquid/solid mixture carry a negative surfacecharge and as a result a positive charge is induced on the liquidmolecules immediately adjacent to such particles. This effectively givesthe liquid, usually water in the cases of aqueous mixtures, a positivecharge. When an electrical field is established between two separatedelectrodes which are buried in the slurry the solid particles will notundergo appreciable movement because of their relatively close packing,but the water, on the other hand, will be carried towards the negativelycharged cathode due to the viscous drag of the migrating positive ionsas they move towards the cathode. Such movement is calledelectro-osmotic flow.

As electro-osmotic flow resulting from electro-osmosis is relativelyindependent of the pore size of the solid particles of the solid/liquidmixture undergoing electro-osmosis, this technique is applicable to awide range of size distributions of the solid particles.

Provided that a current can be induced through the solid/liquid mixture,the rate of liquid movement through the mixture and hence through thesolid particles will be increased above that which can be achieved undergravitational force alone. The rate of liquid movement achieved isdependent on the size of the potential difference applied, the electrodeconfiguration and the separation between the electrodes. Thus, for agiven electrode configuration and a given electrode separation theamount of liquid flow through the mixture can be adjusted by changingthe size of the potential difference applied to the mixture.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method oftreating a solid material containing a substance of economic importanceto recover or extract the substance, the method comprising forming aconducting fluid/solid mixture of the solid material to be treated witha fluid which is at least partially conducting whereby at least some ofthe substance of economic importance is dissolved in the fluid, applyinga potential difference between at least two different electrodes incontact with the fluid/solid mixture at spaced apart locations in orderto import opposite charges to the respective electrodes, attractingfluid containing the dissolved substance of economic importance to oneof the electrodes, removing the fluid from the area surrounding or inthe vicinity of the one electrode, and treating the fluid containing thesubstance of a economic importance so as to recover the substance.

Typically, the material of economic importance is a precious metal, moretypically gold, silver, copper or the like and most preferably gold.

Typically, the fluid is a solution, more typically an aqueous solutionwhich acts as a leaching or irrigating solution. Typically, the leachingsolution contains a material capable of dissolving gold, such as forexample, cyanide.

Typically, the process of the present invention can be applied to eithermaterial which has not been treated previously, i.e. virgin materialmore or less as mined or extracted from the earth, or to material whichpreviously has been treated by one or other existing process which hasproved to be either inefficient or otherwise not capable of recoveringall the gold from the ore resulting in significant amounts of gold beingleft behind in the ore.

Typically, where the material has been leached previously it is possiblethat the gold exists in a form which is capable of being directlydissolved by or soluble in the dissolving fluid and hence the additionof any chemicals to effect dissolution of the gold in a pretreatmentprocess is not required. However, other pretreatment processes may bepossible.

Typically, the present invention can be carried out at any temperaturefrom ambient temperature upwards and is effective at temperaturesbetween the melting point and boiling point of the liquid present usedto extract or dissolve the gold, typically becoming more effective withincreasing temperature within that range.

Typically, the potential difference applied across the slurry formedfrom adding the liquid to the gold bearing material could be in therange from less than a volt to several hundred volts, depending on thespecific application. Typically, power consumption of applying thepotential difference will be within the range 0.1 to 100 Kilowatt-Hrsper tonne of solid material being treated. More typically, the currentpassing through the solid/liquid mixture is about 1 ampere per squaremeter. More typically, the voltage of the potential difference appliedto the slurry is adjusted so as to achieve the required current.

Leach times can vary enormously but typically are anything from aboutten days to about one year or more.

Typically, the selection of optimum conditions of leach time andpotential difference applied, both of which effect power consumption,given the electrical properties of the material being treated, are madeon the basis of the economics of each individual situation so as toachieve maximum benefits.

Typically, the electrode arrangement used in the practice of the processof the present invention can be readily adapted so as to take advantageof the use of electrophoresis to assist with the settling of fineparticles. Electrophoresis is the term used to describe the migration ofsmall electrically charged particles through a stationary liquid.

Typically, the slurry produced from adding the leach liquid to the goldbearing ore which has been comminuted often contains 40 to 70% liquidand it is often necessary to recycle a major portion of this liquid fora variety of reasons which include, the cost of liquid supply, the costof liquid storage and to reduce the harmful effects of liquid disposalon the environment. The use of electrophoresis can greatly assist in therapid recycling of liquid which results in savings and less damage tothe environment.

Typically, the polarity of the electrodes used for electro-osmosis canbe reversed either at the end of or during the process of the presentinvention. Reversal of the polarity can be a single occurrence or may bedone periodically. Settling of the solids in the slurry will be greatlyenhanced by polarity reversal which will result in a clear liquid beingproduced near the negatively charged electrode. This liquid is thenavailable for recycling.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the following examples and accompanying drawings in which

FIG. 1 is a flowsheet of one example of treating ore using the method ofthe present invention as described in Example 1.

FIG. 2 is a schematic view of one form of a leach pond used in oneembodiment of the process of the present invention.

FIG. 3 is a flowsheet of a further example of treating ore using themethod in accordance of the present invention as described in Example 2.

FIG. 4 is a schematic view of one form of a heap treated by one form ofthe process of the present invention.

FIG. 5 is a schematic view of one form of the cathode arrangementemployed in the process of the present invention.

FIG. 6 is a schematic view of one arrangement of the electrodes used inthe process of the present invention.

FIGS. 7(a), 7(b), 7(c) and 7(d) are schematic views of differentarrangements of electrodes in leaching ponds for carrying out the methodof the present invention.

FIGS. 8(a), 8(b), 8(c) and 8(d) are schematic views of differentconfigurations of individual electrodes or sets of electrodes used inthe process of the present invention for treating materials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

With particular reference to FIG. 1, there is shown a flow chart inwhich a gold bearing ore, denoted by reference numeral 1 having thecharacteristics and properties listed below is treated by one form ofthe method of the present invention.

Description of Ore The material to be treated comprises a MineableResource of 500.000 tonnes having on average 5 grams of gold per tonneof the mineable resource.

The Specific Gravity of the mineable resource is about 2.6 and theLiberation Size of the particles from which gold can be recovered isabout 50 micrometers

The Characteristics of the Ore to be treated include the following:

Extremely slow settling rate

Settled solids content 60%

Settled solids mass contains 1.5 tonnes of solids per cubic meter.

Recovery of gold by cyanidation in 30 days is 75 of the total amount ofgold present in the ore.

The process of Example 1 is illustrated in the Flowchart of FIG. 1 andcomprises the steps of taking the material to be treated, i.e. the ore,1 and passing the ore 1 to a crushing circuit, generally denoted as 2,which consists of a three stage crushing operation using a primary jawcrusher, a secondary cone crusher, and a tertiary cone crusher operatingin closed circuit with a vibrating screen. After passing through thecrushing circuit 2 the ore is reduced to a maximum particle size ofabout 10 millimeters on average.

The crushed ore from the crushing circuit 2 is then fed to a grindingcircuit 4 comprising a two stage ball milling circuit consisting of twoball mill/cyclone circuits in series. The grinding circuit 4 reduces themaximum particle size of the ore to 60 micrometers.

Lime 3 in euitable form and Sodium Cyanide 5 are added to the grindingcircuit, together with a suitable solvent, such as water 7, to form aslurry of the ore being treated.

The product from grinding circuit 4, which is in the form of a slurrycontaining 30% solids, by weight is then directed to a leaching pond,generally denoted as 6, which comprises three separate leaching ponds6a, 6b, 6c, so that the slurry is admitted to one of ponds 6a, 6b or 6cin which a leaching cycle is conducted. The general arrangement of oneof the ponds 6a, 6b or 6c is illustrated in FIG. 2. However, otherarrangements of the leaching pond are possible.

Leach Pond 6 comprises in use a clear water zone 12 located towards theupper surface of pond 6, a settling zone 11, a settled solids zone 10and a drainage zone 20 located at the base of the pond 6.

With particular reference to FIG. 2, the leach cycle in pond 6a and inthe other ponds 6b and 6cconsists of three stages which are filling,leaching and drainage, and each stage is of 28 days duration. During thefilling stage a total of 11200 tonnes of ore, in a slurry containing 30%solids, is fed to the pond from grinding circuit 4. In each day 400tonnes of solids reaches the settled solids zone 10 of leach pond 6a,together with 400 cubic meters of entrained water. At the same time 533cubic meters of water reach the clear water zone 12 add enter dischargewell 14 to be pumped away to pregnant pond 16 by means of pump 18.

Within the settled solids zone 10 compaction of the solids bed occurswhich squeezes some of the entrained water out of zone 10 into thedrainage zone 20 for removal via well 14 to pregnant pond 16 by means ofpump 18. By the end of the first 28 day period the settled solids zone10 contains 60% solids which means that a total of 3733 cubic meters ofinitially entrained water has been removed to the pregnant pond 16.

As the above described processes are occurring, dissolution of the goldinitially contained in the ore will also be occurring wherever the oreis in contact with the liquid slurry.

After the filling stage is completed in first pond 6a, fresh slurry fromgrinding circuit 4 is directed to the second leaching pond 6b and theleaching process commences in the first pond 6a.

During the second 28 day period of the process which is the 28 dayleaching stage in pond 6a barren leaching solution 25 will be pumpedfrom barren leaching solution pond 28 (to be described in detail later)and drawn through the bed of ore solids in the settled solids zone 10through to drainage zone 20, which comprises a bed of screen aggregate.The leaching solution is drawn to the drainage zone 20 through settledsolids zone 10 by applying a potential difference between a firstelectrode 22 located at the top of pond 6a and a second electrode 24located at the bottom of the settled solids zone 10 of pond 6a.

During the second 28 day period which is the 28 day leach period of pond6a a total of 11200 cubic meters of initially barren leach solution ispassed through the settled solids zone 10 dissolving gold and picking updissolved gold on the way. The gold containing leach solution is thendirected to pregnant pond 16 via drainage zone 20 and well 14 by meansof pump 18.

At the termination of the leach stage conducted in the first pond 6a thedrainage stage will then commence in pond 6a while a leach stage willcommence in the second pond 6b, and a filling stage will commence in thethird pond 6c. Thus, the three ponds 6a,6b,6c all operate out of phase,with a different stage of the three stage process occurring in each pondat any one given time. Furthermore, each of the three stages is operatedin sequence in cycles.

During the drainage stage the flow of barren leach solution 25 frombarren pond 28 to the leach pond 6a is stopped but the flow of leachsolution through the settled solids zone 10 continues under the effectof the potential difference applied between the electrodes 22,24 todissolve and transport the gold.

During the drainage stage the solids content of the settled solid zone10 is increased to 80%. This results in a further 4667 cubic meters ofleach solution being obtained via the drainage zone 20 and well 14.

Table 1 shows the inputs and outputs to each of the individual stages,and the accumulative amounts of each material.

The liquid movement induced in the leaching stage by the application ofa potential difference between the respective electrodes in accordancewith the present invention is sufficient over depths of up to about 5meters, although the potential difference between the electrodes willneed to be increased as the depth of the heap being treated increases.

When the depth of the settled zone 10 reaches 5 meters, which occursafter the sixth drainage stage, a new drainage layer and new electrodeswill be required.

At any point in time, with the exception of the first and last fifty sixdays of the life of the operation there is one of the three stages inprogress in one of the leach ponds 6 at all times. Consequently the flowand grade (amount of dissolved gold) of pregnant solution is constantover the three ponds in operation. During the first two and last two 28day periods the flow of pregnant solution is low since all three pondsare not in operation. Table 2 shows the periodic output from the threeponds combined while Table 3 shows the annual and overall performance.

All pregnant solution containing dissolved gold taken from each of theleach ponds 6a,6b,6c is directed to the pregnant solution pond 16 whichacts as a feed source for a set of adsorption columns containingactivated carbon in a carbon adsorption circuit 26. In carbon adsorptioncircuit 26 gold is removed from the gold containing pregnant solution bybeing absorbed onto the activated carbon thus producing a solutionexiting from the circuit 26 which is substantially free of gold i.e. abarren solution. The barren exit solution from carbon adsorption circuit26 is directed to barren pond 28 from which leach solution 25 is drawnfor delivery to the leach ponds 6 during further leach cycles.

The carbon, having become loaded with gold, is periodically removed fromthe adsorption columns of the carbon adsorption circuit 26, and the goldis stripped from the carbon by way of elution with a hot caustic/cyanidesolution and then returned to the adsorption circuit 26 ready to removefurther gold from further lots of pregnant solution.

The caustic/cyanide solution containing the gold stripped from thecarbon is then passed through an electrolytic cell. 30 where the gold isplated out onto a steel wool electrode. When fully loaded with gold, theelectrode is then calcined add the resulting calcine smelted to producegold ore. The gold in a more or less purified form can then be refinedfrom the gold ore hence recovering the gold from the originalgold-bearing ore which processes are denoted by 31 in FIG. 1.

EXAMPLE 2

With particular reference to FIG. 3, there is shown a further flow chartin which a gold bearing ore 1 having the characteristics and propertieslisted below is treated by another form of the method of the presentinvention.

Description of Ore

The ore to be treated comprises a Mineable Resource of some 1,000,000tonnes having on average 2 grams of gold per tonne of the mineableresource.

The Specific Gravity of the ore is about 2.6 and the characteristics ofthe Crushed Ore to be treated are as follows:

High fines content

Low compressive strength

Bulk density 1.5 tonnes of solids per cubic meter

Angle of repose 35° C.

Heap leachable, with agglomeration to give 80% recover

in 90 days.

The general flowsheet of this form of the treatment is shown in FIG. 3which comprises a crushing circuit 2 consisting of a primary jaw crusherfollowed by an impact crusher operating in closed circuit with avibrating screen. Lime 3 is added to the feed of the secondary crusherof the crushing circuit 2.

The crushed ore exiting from crushing circuit 2 is transported, such asfor example by truck to a heap leach pad 32 where it is stacked andlevelled by a bulldozer or similar to form a heap or pile of crushed oreto a height of 10 meters.

When a heap containing one month's delivery of crushed ore has beenobtained, barren solution 25 containing sodium cyanide, acting as anirrigating or leaching solution is pumped onto the top of heap 32 at arate of about 10 liter/hour per square meter of heap area. Theirrigating solution 25 is drawn to the base of the heap by gravity,aided by the method of the present invention.

After a period of about 60 days irrigation of the heap is discontinuedand the heap drained, again assisted by the method of the presentinvention.

When the entire heap area has been covered, to a height of 10 meters, asecond lift or heap of ten meters is built upon the first heap. This isrepeated until a total of five lifts or heaps have been built into asingle stack.

The nature of the ore being treated in this example is such that,without using the method of the present invention, heap leaching wouldonly be possible after agglomeration and careful handling of theagglomerated product. Furthermore, the stack height is limited by thelow strength of the ore.

The general arrangement of a single ore heap or stack is shown in FIG.4.

Irrigation of the heap 40 is effected by barren irrigating or leachingsolution 25 being pumped from a suitable reservoir such as barren pond28 through pipes 42 to exit through nozzles 44 located above the toplevel of the heap 40. The irrigation of heap 40 is maintained for athree month period. A potential difference is applied to the heap bylocating an anode or anodes 46 at or towards the top of the heap 40 anda cathode or cathodes 48 at the base of the heap near to where thepregnant solution exits from the heap. Anode 46 and cathode 48 are eachin electrical connection with a direct current supply 50 and inelectrical connection with each other through heap 40 to complete anelectrical circuit. After the initial wetting of the ore the amount ofpregnant solution exiting from the base of the heap is about the same asthat irrigating the heap.

At the end of the about three months irrigation period the flow ofirrigating solution 25 to the heap from pipes 42 is discontinued but thepotential difference between the electrodes 46,48 maintained. Thiscauses the irrigation solution to flow through and exit from the heap.After a period of about one month for drainage the potential differencebetween the electrodes is disconnected and the heap allowed to rest.

About one year after the drainage stage has been completed the potentialdifference is reapplied and the heap irrigated with irrigating solution25 for a further period of about three months. During this furtherperiod any gold which has dissolved into the irrigating solution duringthe year of inactivity is recovered.

Table 4 shows the performance which is achieved on each one monthssample of ore at 10 day intervals from the commencement of the furtherthree month period. Table 5 shows the performance overall.

The process for extraction of the gold from the pregnant solution, andthe subsequent production of gold is carried out as described in example1.

EXAMPLE 3

In this form of the method of the present invention tailings resultingfrom a cyanidation operation, which had been completed some yearspreviously were left in a dump. The dimensions of the dump are 200 by200 by 10 meters deep and the dump contains some 600,000 tonnes oftailings.

The tailings contain one gram of gold per tonne on average, of which 0.6grams is water soluble.

The material in the dump is extremely fine and virtually impermeable towater flow by gravity. Accordingly, the present invention can be used toextract the water soluble gold, in-situ, by the following method.

With particular reference to FIG. 5, holes are drilled in the materialin the dump at regularly spaced apart intervals on a 20 by 20 meter gridpattern and each hole is lined with a slotted pipe 58, inside of whichis placed an electrode. One arrangement of one electrode, being acathode is as illustrated in FIG. 5.

The electrodes in the holes are either anodes or cathodes. One form ofthe cathode arrangement 60 consists of a solid tube of metal 62centrally axially located in the hole surrounded by activated carbon 64which is held in baskets 66 to facilitate removal of the gold bearingcarbon.

Anodes 68 are also located in holes cased in slotted pipe 58 similar tothe cathodes. The arrangement of the positioning of the anodes, andcathodes with respect to each other is shown in FIG. 6.

The holes within which anodes 68 are positioned are kept full withbarren solution which, under the effect of the electrical field producedby the potential difference applied to the electrodes, moves through thematerial in the dump of being treated towards the cathodes.

As the barren solution moves through the material in the dump towardsthe cathodes it picks up or otherwise leaches or dissolves gold from thematerial to form a gold containing leaching solution. As the goldcontaining solution reaches cathodes 62 the gold contained in theleaching solution is adsorbed by the activated carbon 64 held in baskets66 surrounding the cathodes 62. Periodically, baskets 66 containing thecarbon 64 are removed and the carbon treated using the process asdescribed in example 1 to extract the gold.

The solution after passing through the activated carbon 64 issubstantially free of gold and may then be pumped from cathodes 62 to apond from which it will be returned to the anodes 68 to continue theprocessing of the material in the dump.

At any point in time a total of 36 cathodes, and the correspondingnumber of associated anodes are in use. The electrical potential isapplied for a total of 120 days but after 100 days the recycling ofsolution to the anodes is stopped. During the last 20 days of the leachperiod the dump area surrounding the cathode is "dried out" enabling theactivated carbon to be removed for further processing.

Table 6 shows the performance which has been achieved by one cathodewhile Table 7 shows the overall performance which has been achieved.

Although one embodiment of the present invention has been described indetail, other embodiments are possible depending on circumstances. Someof the other embodiments of the arrangement of the electrodes are shownin FIGS. and 9.

In FIGS. 7(a) to 7(d), there is shown different arrangements of theelectrodes within the leach pond for use in carrying out the method ofthe present invention to facilitate gold dissolution.

In these embodiments, there is a dam wall or similar 80 containingliquid/solid mixture 82 in which is located anodes 84 and cathodes 86.The anodes and cathodes may be arranged as shown or may take any otherarrangement.

Pregnant solution containing dissolved gold is drawn off via pipe 88 oroverflow weir 90.

FIGS. 8(a) to 8(d) are shown four different configurations of electrodescomprising anode 84 and cathode 86 in the leach pond. In someembodiments, as shown in FIG. 8(c), cathode 86 is surrounded byabsorbent material 92 for the dissolved gold in the leach solution.

The benefits and advantages of the present invention over existingmethods relates to the low capital cost of setting up the plant fortreatment of the ore materials and the operating costs involved inactually treating the ore material and the applicability of the presentinvention to materials having virtually any settling and sizecharacteristics.

The lower capital and operating costs involved with the practice of thepresent invention means that resources which cannot be economicallyexploited using existing techniques can now ben treated and the overallprocess becomes economically viable.

The present invention could also be used as an adjunct to existingtechniques to ensure that available resources are used to the maximumextent, and that gold is recovered more efficiently.

A further advantage of the present invention is that liquids used in theirrigation as leaching of the ore materials can be removed to a greaterextent than is possible by existing methods. This greatly facilitatesthe rehabilitation of waste storage facilities.

                                      TABLE 1                                     __________________________________________________________________________    INDIVDUAL STAGES - EXAMPLE 1                                                                     GOLD       PREGNANT SOLUTION                                    SOLIDS                                                                             SOLN.                                                                             %    UN DSLV                                                                             DSLVD                                                                              VOL  GOLD                                                                              GRADE                                  STAGE                                                                              tonnes                                                                             M.sup.-3                                                                          SOLIDS                                                                             OZS   OZS  M.sup.-3                                                                           OZS ppm                                    __________________________________________________________________________    FEED 11200                                                                              26133                                                                             30.0%                                                                              1800   0     0   0  0.00                                   FILL 11200                                                                              7467                                                                              60.0%                                                                              942   620  18667                                                                              239 0.40                                   LEACH                                                                              11200                                                                              7467                                                                              60.0%                                                                              202   468  11200                                                                              891 2.48                                   DRAIN                                                                              11200                                                                              2800                                                                              80.0%                                                                               17   279   4667                                                                              374 2.49                                   __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                        PERIOD OUTPUT FROM LEACH PONDS - EXAMPLE 1                                                          GOLD                                                             SOLN.        DSLVD    GRADE                                          PERIOD   M.sup.-3     OZS      ppm                                            ______________________________________                                         1       18667         239     0.40                                            2       29867        1130     1.18                                           3-44     34533        1504     1.35                                           45       27867        1419     1.58                                           46       11867         947     2.48                                           47        3000         240     2.49                                           ______________________________________                                    

                                      TABLE 3                                     __________________________________________________________________________    ANNUAL AND OVERALL PERFORMANCE - EXAMPLE 1                                               FEED     GOLD     PROGRESSIVE                                                 GRADE                                                                              GOLD                                                                              RECOVERED                                                                              RECOVERY                                         YEAR TONNES                                                                              g/t  OZS OZS      %                                                __________________________________________________________________________    1    145600                                                                              5.0  23405                                                                             17911    76.5%                                            2    145600                                                                              5.0  23405                                                                             19550    80.0%                                            3    145600                                                                              5.0  23405                                                                             19550    81.2%                                            4     63200                                                                              5.0  10159                                                                             10125    83.5%                                            TOTAL                                                                              500000                                                                              5.0  80376                                                                             67136    83.5%                                            __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    PERFORMANCE ON ONE MONTHS ORE - EXAMPLE 2                                                          REMAINING                                                       PREGNANT SOLUTION                                                                           U/D DISVL.                                                                             EXTRACTION                                             VOL  GRADE                                                                              GOLD                                                                              GOLD                                                                              GOLD %    %                                          DAY    M.sup.-3                                                                           ppm  ozs ozs ozs  DISLV                                                                              REC.                                       __________________________________________________________________________    10      107 1.79  6  809 107  24.5%                                                                               0.6%                                      20      2729                                                                              1.80 158 686 117  36.0%                                                                              14.7%                                      30      5739                                                                              1.67 308 573 101  46.5%                                                                              28.8%                                      40      8748                                                                              1.54 434 479 89   55.3%                                                                              40.5%                                      50     11757                                                                              1.42 538 401 79   62.6%                                                                              50.2%                                      60     14766                                                                              1.32 625 335 70   68.7%                                                                              58.4%                                      70     17776                                                                              1.22 699 280 63   73.9%                                                                              65.2%                                      80     20785                                                                              1.14 760 234 57   78.1%                                                                              70.9%                                      90     23794                                                                              1.06 811 222 52   79.3%                                                                              75.6%                                      100    24767                                                                              1.06 841 209 34   80.5%                                                                              78.5%                                      110    25515                                                                              1.04 855 196 33   81.7%                                                                              79.8%                                      120    26088                                                                              1.03 860 186 39   82.7%                                                                              80.3%                                      RELEACH                                                                              10000                                                                              1.34 431  54 27   12.3%                                                                              13.5%                                      TOTAL  36088                                                                              1.11 1291                                                                               54 27   95.0%                                                                              93.7%                                      __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________    OVERALL PERFORMANCE - EXAMPLE 2                                                           FEED  PREGNANT SOLUTION                                                                            GOLD                                               CRUSH GRADE GOLD                                                                              VOL   GRADE                                                                              PROD.                                        YEAR  TONNES                                                                              g/t   ozs M.sup.-3                                                                            ppm  ozs                                          __________________________________________________________________________    1     200000                                                                              2.0   12860                                                                             131347                                                                              1.09  4592                                        2     200000                                                                              2.0   12860                                                                             179835                                                                              1.60  9223                                        3     200000                                                                              2.0   12860                                                                             202743                                                                              1.87 12212                                        4     200000                                                                              2.0   12860                                                                             202743                                                                              1.87 12212                                        5     200000                                                                              2.0   12860                                                                             202743                                                                              1.87 12212                                        6                      71396                                                                              3.32  7619                                        7                      13745                                                                              4.96  2192                                        TOTAL 1000000                                                                             2.0   64300                                                                             1004552                                                                             1.87 60261                                        __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    PERFORMANCE AT CATHODE DURING LEACH - EXAMPLE 3                               PREGNANT SOLUTION                                                             DAILY         PROGRESSIVE                                                                            ORE REMAINING                                              VOL  GRADE                                                                              VOL GRADE                                                                              GOLD      GRADE                                                                              GOLD                                    DAY M.sup.-3 /day                                                                      ppm  M.sup.-3                                                                          ppm  ozs TONNES                                                                              g/t  ozs                                     __________________________________________________________________________                               6000  1.00 192.9                                   20  52.9 1.70 1059                                                                              1.70  57.9                                                                             6000  0.70 135.0                                   40  52.9 0.85 2118                                                                              1.27  86.8                                                                             6000  0.55 106.1                                   60  52.9 0.43 3176                                                                              0.99 101.3                                                                             6000  0.48 91.6                                    80  52.9 0.21 4235                                                                              0.80 108.5                                                                             6000  0.44 84.4                                    100 52.9 0.11 5294                                                                              0.66 112.1                                                                             6000  0.42 80.8                                    120 26.5 0.11 5824                                                                              0.61 113.9                                                                             6000  0.41 79.0                                    __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    OVERALL PERFORMANCE - EXAMPLE 3                                                      PREGNANT     RECOVERY OF                                                      SOLUTION     REMAINING IN DUMP                                                VOL GRADE                                                                              GOLD    SOLUBLE                                                                             GRADE                                                                              GOLD                                       QUARTER                                                                              M.sup.-3                                                                          ppm  ozs GOLD                                                                              GOLD  g/t  ozs                                        __________________________________________________________________________    1      135794                                                                            0.85 3730                                                                              11.6%                                                                             19.3% 0.88 17052                                      2      154853                                                                            0.68 3399                                                                              10.6%                                                                             17.6% 0.78 15013                                      3      156838                                                                            0.49 2478                                                                               7.7%                                                                             12.8% 0.70 13526                                      4      163985                                                                            0.53 2801                                                                               8.7%                                                                             14.5% 0.61 11845                                      YEAR 1 611471                                                                            0.63 12408                                                                             38.6%                                                                             64.3% 0.61 11845                                      1      162794                                                                            0.71 3703                                                                              11.5%                                                                             19.2% 0.50  9624                                      2       96606                                                                            0.58 1811                                                                               5.6%                                                                              9.4% 0.44  8537                                      YEAR 2 259400                                                                            0.66 5514                                                                              17.2%                                                                             28.6% 0.44  8537                                      TOTAL  870871                                                                            0.64 17922                                                                             55.7%                                                                             92.9% 0.44  8537                                      __________________________________________________________________________

The described arrangement has been advanced by explanation and manymodifications may be made without departing from the spirit and scope ofthe invention which includes every novel feature and novel combinationof features hereindisclosed.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described. It is understood that the invention includes allsuch variations and modifications which fall within the spirit andscope.

We claim:
 1. A method of treating a solid material containing asubstance of economic importance to recover or extract the substancefrom the material, said method comprising forming a conducting mixtureof the solid with a liquid so that at least some of the substance ofeconomic importance is dissolved in the liquid, said liquid being atleast partially conducting, applying an electrical potential differencebetween at least two electrodes in contact with the mixture at spacedapart locations so as to impart opposite charges to the respectiveelectrodes thereby attracting the liquid containing the dissolvedsubstance to one of the electrodes, removing the attracted liquid froman area surrounding or from a vicinity of the one electrode, andtreating the removed liquid containing the substance of economicimportance so as to recover or extract the substance.
 2. The method ofclaim 1 in which the liquid is an aqueous solution in the form of aleaching or irrigating solution.
 3. The method of claim 2 in which theleaching or irrigating solution dissolves at least partially thesubstance of economic importance.
 4. The method of claim 3 in which theleaching or irrigating solution contains cyanide.
 5. The method of claim1 in which the solid material being treated is either virgin material oris material that has been previously treated to recover the substance ofeconomic importance.
 6. The method of claim 1 in which the potentialdifference applied across the electrodes is in the range of less thanone volt to several hundred volts.
 7. The method of claim 1 in which thestep of applying the potential difference across the electrodes resultin a power being consumed in the range of 0.1 to 100 kilowatt-hours pertonne of solid material being treated.
 8. The method of claim 1 in whichthe step of applying the potential difference across the electrodescauses a current to pass through the conducting mixture wherein thecurrent is about 1 ampere per square meter.
 9. The method of claim 1 inwhich the leaching of the solid material being treated takes from 10days to one year or more.
 10. The method of claim 1 in which thesolid/liquid mixture comprises 40% to 70% liquid by weight.
 11. Themethod of claim 1 further comprising recycling the liquid through thesolid material being treated.
 12. The method of claim 1 in which theelectrodes comprise one or more anodes interconnected together and oneor more cathodes interconnected together.
 13. The method of claim 1 inwhich one of the electrodes is more or less surrounded by absorbentmaterial for absorbing the material of economic importance.
 14. Themethod of claim 1 in which the substance of economic importance isselected from the group consisting of gold, silver and copper.
 15. Themethod of claim 1 further comprising pretreating the solid material tofacilitate dissolution of the substance of economic importance.
 16. Themethod of claim 1 in which the electrodes are arranged so as to takeadvantage of the use of electrophoresis to assist with settling of fineparticles.
 17. The method of claim 1 further comprising reversingpolarity of the electrodes either as a single occurrence orperiodically.